The present invention relates generally to mounting optical elements in optical modules and, more particularly, to technique for mounting an optical element to a structural element using an intermediate element.
When designing optical mounts for optimized thermal performance, such as is necessary in Dense Wavelength Division Multiplexing (DWDM) applications, it is critical that thermal stresses are directed to interfaces which are not damaged by these thermal stresses. For example, when bonding fused silica directly to stainless steel (i.e., optical element material to substrate material) using epoxy, it is frequently seen that the fused silica will fracture during an elevated temperature and reduced temperature bond curing cycle.
Specifically, consider the example of a glass optical element, such as an optical prism having a width of 25 millimeters, that must be mounted to a steel substrate. Assuming that the optical prism has a coefficient of thermal expansion (CTE) of 5xc3x9710xe2x88x926 and the steel substrate has a CTE of 10xc3x9710xe2x88x926, there exists a CTE difference of 5xc3x9710xe2x88x926 between the optical prism and the steel substrate. If the optical prism is mounted to the steel substrate directly by some means such as a thin epoxy layer at room temperature (i.e., 25xc2x0 C.), then at 85xc2x0 C. there will exist a expansion differential across the width of the optical prism of 7.5 microns. This expansion differential is calculated as the product of the length of the interface between the optical prism and the steel substrate (i.e., 25 mm), the CTE difference between the optical prism and the steel substrate (i.e., 5xc3x9710xe2x88x926), and the change in temperature (i.e., 60xc2x0 C.). Since the optical prism and the steel substrate are directly bonded, this expansion differential must be taken up by additional stress in the optical prism, the steel substrate, and the epoxy. If the stress is too high, the optical prism and/or epoxy may crack, or the epoxy may delaminate. These are obviously undesirable conditions.
Various methods have been proposed for reducing or mitigating temperature-induced expansion stresses between materials having different coefficients of thermal expansion. One method is through the use of small-area interfaces where the total stress is small. However, this method typically results in less secure mountings as less area is available for such mountings. Another method is through the use of flexure designs, whereby material flexes through thermally-induced stress in a controlled manner. However, this method requires that flexures be designed into mounting surfaces.
In view of the foregoing, it would be desirable to provide a technique for mounting an optical element to a structural element which overcomes the above-described inadequacies and shortcomings in an efficient and cost effective manner.
According to the present invention, a technique for attaching an optical element to a structural element is provided. In one particular exemplary embodiment, the technique may be realized as an apparatus for attaching an optical element to a structural element. Such an apparatus may comprise an optical element formed of a material having a first coefficient of thermal expansion value, a structural element formed of a material having a second coefficient of thermal expansion value, and an intermediate element formed of a material having a third coefficient of thermal expansion value that is between the first coefficient of thermal expansion value and the second coefficient of thermal expansion value. The intermediate element is disposed between the optical element and the structural element such that thermal stress between the optical element and the structural element are transferred to the intermediate element.
In accordance with other aspects of this particular exemplary embodiment of the present invention, the size of the intermediate element may beneficially be smaller than the size of the optical element for reducing thermal stress between the intermediate element and the optical element.
In accordance with further aspects of this particular exemplary embodiment of the present invention, the yield strength of the intermediate element may beneficially be greater than the yield strength of the optical element.
In accordance with additional aspects of this particular exemplary embodiment of the present invention, the intermediate element may beneficially be secured to the optical element by an adhesive, a metal joint, and/or welding. Similarly, the intermediate element may beneficially be secured to the structural element by an adhesive, a metal joint, and/or welding. Also, the intermediate element may beneficially comprise a single material or material layer or multiple stacked materials or material layers. For example, the intermediate element may beneficially be a first intermediate element, and the apparatus may further beneficially comprise a second intermediate element formed of a material having a fourth coefficient of thermal expansion value that is between first coefficient of thermal expansion value and the second coefficient of thermal expansion value, wherein the second intermediate element is disposed between the optical element and the first intermediate element. Alternatively, the intermediate element may beneficially be a first intermediate element, and the apparatus may further beneficially comprise a second intermediate element formed of a material having a fourth coefficient of thermal expansion value that is between first coefficient of thermal expansion value and the second coefficient of thermal expansion value, wherein the second intermediate element is disposed between the structural element and the first intermediate element.
In accordance with still other aspects of this particular exemplary embodiment of the present invention, the optical element may beneficially comprise an optical lens, an optical prism, or an optical diffraction grating. Accordingly, the optical element may beneficially be formed of glass, ceramic, plastic, and/or composite material.
In accordance with still further aspects of this particular exemplary embodiment of the present invention, the structural element may beneficially be formed of a metal, ceramic, plastic, and/or composite material. Similarly, the intermediate element may beneficially be formed of a metal, ceramic, plastic, and/or composite material.
The present invention will now be described in more detail with reference to exemplary embodiments thereof as shown in the appended drawings. While the present invention is described below with reference to preferred embodiments, it should be understood that the present invention is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional implementations, modifications, and embodiments, as well as other fields of use, which are within the scope of the present invention as disclosed and claimed herein, and with respect to which the present invention could be of significant utility.